Question

Diazotization of \(2,4-\) dinitroaniline in aqueous solution is accompanied by some conversion to phenols in which a nitro group is replaced by a hydroxy group. Give a reasonable mechanism for this reaction.

Short answer

In the diazotization of 2,4-dinitroaniline, the primary amine group (NH2) reacts with nitrous acid (HNO2) in the presence of an acid like hydrochloric acid (HCl) to form a diazonium salt. Subsequently, one of the nitro (NO2) groups undergoes substitution with a hydroxy (OH) group due to the electron-rich diazonium nitrogen initiating the reaction, subsequently resulting in the leaving of the NO2 group with the diazonium salt N2+. Finally, the remaining pair of electrons on the central carbon reacts with water or another nucleophile, leading to the formation of phenol, where one of the nitro groups has been replaced with a hydroxy group.

Step-by-step solution

Write the structure of 2,4-dinitroaniline

First, let's draw the structure of 2,4-dinitroaniline: ``` NO2 NH2 | | H - C-C - C-C - H | | H - C-C - C-N - O2 | NO2 ``` 2,4-dinitroaniline has an amino (NH2) group and is substituted with two nitro (NO2) groups on the benzene ring.

Diagram the diazotization process

During diazotization, the primary amine group (NH2) is converted into a diazonium salt (N2+). The reaction takes place in the presence of nitrous acid (HNO2) and an acid like hydrochloric acid (HCl). The chemical equation for the reaction is: 2,4-dinitroaniline + HNO2 + HCl → 2,4-dinitroaniline diazonium salt + H2O ``` H NO2 HNO2(HCl) Cl | ----- | H - C-C -N2+ - - - C-C - H | | | H - C-C - C=N - - - O2 | | NO2 H ```

Provide a mechanism for the replacement of a nitro group by a hydroxy group

This step will involve demonstrating how one of the nitro groups undergoes a substitution reaction to be replaced with a hydroxy (OH) group as part of the phenol. 1. The electron-rich diazonium nitrogen from the diazonium salt initiates the reaction by attracting a nitro group to get electron density from the diazonium salt N2+. ``` Cl X NO2 | | | H -C-C-N2+-C-C-H | | | H -C-C-W-C-N=O | | NO2 H ``` 2. As a result, one of the NO2 groups leaves as a leaving group with the diazonium salt N2+, and the carbon position previously occupied by the NO2 groups gains a positive charge. ``` Cl NO2 | | H -C-C+-C-C-H | | H -C-C Y-C-N=O | | NO2 H ``` 3. The remaining pair of electrons on the central carbon reacts with water or another nucleophile, resulting in the addition of a hydroxy group (OH) and the formation of a phenol compound. ``` Cl | H -C-C -C-C-H | | H -C-C -C-C-N=O | | NO2 OH ``` The final product is a phenol where one of the nitro groups has been replaced with a hydroxy group.

Key concepts

2,4-dinitroaniline

2,4-dinitroaniline is a chemical compound that serves as the starting material in the reaction of diazotization. This compound features a benzene ring with three significant substituents:

• An amino group (NH2) in its structure, which is necessary for forming diazonium salts.
• Two nitro groups (NO2) on the benzene ring, specifically at the 2nd and 4th positions relative to the amino group.

The structure of 2,4-dinitroaniline is not simply for decoration; each group plays a crucial role in the reaction process.

• The amino group is the focal point for chemical modification through diazotization.
• The nitro groups contribute to the electron dynamics of the molecule, influencing its chemical behavior.

Understanding the structure of 2,4-dinitroaniline helps us predict its reactivity and understand the mechanism it will participate in.

Diazonium Salt

A diazonium salt is a temporary but pivotal chemical intermediate in the reaction process. When 2,4-dinitroaniline undergoes diazotization, the amino group is transformed into a diazonium ion.
This is achieved using nitrous acid ( HNO2) and a strong acid such as hydrochloric acid (HCl).
Here's what happens during this transformation:

• The amino group (NH2) on the 2,4-dinitroaniline reacts with nitrous acid.
• The reaction results in an unstable diazonium ion ( N2+), while water is released as a byproduct.
• The overall equation can be summarized as: 2,4-dinitroaniline + HNO2 + HCl → 2,4-dinitroaniline diazonium salt + H2O.

This diazonium ion is a powerful intermediate since it can be easily converted to various functional groups. In this mechanism, it plays a catalytic role in the subsequent substitution reaction.

Substitution Reaction

In this reaction mechanism, substitution occurs when a nitro group is removed and replaced by a hydroxyl group, leading to phenol formation. This process involves a series of electron shifts and bond rearrangements:

• The electron-rich diazonium nitrogen initiates the substitution by attracting one of the nitro groups.
• This enables the diazonium ion to "carry away" the nitro group as it breaks its bond with the benzene ring.
• The leaving nitro group departs with its electrons, providing an empty spot (positive charge) for incoming reactions.

The introduction of water or another nucleophile to the positively charged carbon then facilitates the addition of an OH group. Substitution reactions like this often use diazonium salts due to their excellent leaving group characteristics.

Phenol Formation

The ultimate goal of the reaction is to substitute a nitro group with an OH group, forming a phenol. This transformation is essential in various chemical syntheses and has many applications in drug development and material science.

• Replacing a nitro group with a hydroxyl group changes not just the functional group but also the chemical properties of the molecule.
• The phenol produced has a benzene ring, an OH group, and one remaining nitro group.

Through this process, the electron-rich environment enables the OH group to be added. Phenols have distinct properties because of the presence of the OH group. These properties make them versatile intermediates and end points in organic synthesis.